test: add block scoping to test-readline-interface

Use block-scoping in test-readline-interface to avoid side effects and
make tests more modular. (Some contain race conditions and will need to
be moved to the sequential directory if they can't be refactored to
avoid the race condition.)

Backport-PR-URL: #14748
Backport-Reviewed-By: Anna Henningsen <anna@addaleax.net>

PR-URL: #14615
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>